The present invention relates generally to semiconductor packaging, and more particularly to a method of assembling a semiconductor device that includes a heat spreader.
As is well known, integrated circuits are formed on semiconductor dies. Such circuits are formed with multiple layers of materials, e.g., conductors and insulators that interconnect tiny transistors. The dies can include hundreds of thousands and even millions of transistors. Thus, in operation, a substantial amount of heat can be generated by the circuit, especially if the circuit operates at a high voltage. One way of dissipating this heat is to use a heat spreader. A heat spreader, which in its simplest form is a metal plate, conducts heat away from the die so that the circuit does not over heat.
In order to protect the semiconductor dies, the dies are attached and electrically connected to a substrate and then the substrate and die are encapsulated with a plastic material. Interconnections to the die are provided by way of the substrate. One popular external connection is a ball grid array (BGA), which is an array of conductive balls attached to the bottom of the substrate. Wires or metal lines (provided in multiple layers) in the substrate allow for electrical connection between the conductive balls and the pads on the die. One way of assembling a BGA type device is to assemble an array of devices at the same time, which is known as MAP (Molded Array Package) BGA.
However, various process limitations have made it difficult to assemble a MAP BGA package that includes a heat spreader. For example, height variations of the individual heat spreaders will result in uneven mold clamping and mold compound bleeding on top of the heat spreader, and difficulties in mold flow control caused by the use of side gate molding where mold compound must flow across tight spacing, which can cause voids and wire sweep issues (movement or bond wires caused by flow of mold compound during molding). Thus, packages with heat spreaders have been limited to over molded plastic array carrier (OMPAC) type packages in which individual heat spreaders are directly attached to die after the die has been attached and electrically connected to the substrate. However, a problem faced by OMPAC packaging is that when the heat spreader is directly attached to the die before molding, when the mold chase is closed, a clamping force can be exerted on the die, causing the die to crack. Accordingly, it would be advantageous to be able to efficiently assemble MAP devices that include heat spreaders in which the risk of the die cracking was substantially reduced or eliminated.